Measurement of the detective quantum efficiency in digital detectors consistent with the IEC 62220-1 standard: practical considerations regarding the choice of filter material.

As part of a larger evaluation we attempted to measure the detective quantum efficiency (DQE) of an amorphous silicon flat-panel detector using the method described in the International Electrotechnical Commission standard 62220-1 published in October 2003. To achieve the radiographic beam conditions specified in the standard, we purchased scientific-grade ultrahigh purity aluminum (99.999% purity, type-11999 alloy) filters in thicknesses ranging from 0.1 through 10.0 mm from a well-known, specialty metals supplier. Qualitative evaluation of flat field images acquired at 71 kV (RQA5 beam quality) with 21 mm of ultrahigh purity aluminum filtration demonstrated a low frequency mottle that was reproducible and was not observed when the measurement was repeated at 74 kV (RQA5 beam quality) with 21 mm of lower-purity aluminum (99.0% purity, type-1100 alloy) filtration. This finding was ultimately attributed to the larger grain size (approximately 1-2 mm) of high purity aluminum metal, which is a well-known characteristic, particularly in thicknesses greater than 1 mm. The impact of this low frequency mottle is to significantly overestimate the noise power spectrum (NPS) at spatial frequencies < or = 0.2 mm(-1), which in turn would cause an underestimation of the DQE in this range. A subsequent evaluation of ultrahigh purity aluminum, purchased from a second source, suggests, that reduced grain size can be achieved by the process of annealing. Images acquired with this sample demonstrated vertical striated nonuniformities that are attributed to the manufacturing method and which do not appear to appreciably impact the NPS at spatial frequencies > or = 0.5 mm(-1), but do result in an asymmetry in the x- and y-NPS at spatial frequencies < or = 0.2 mm(-1). Our observations of markedly visible nonuniformities in images acquired with high purity aluminum filtration suggest that the uniformity of filter materials should be carefully evaluated and taken into consideration when measuring the DQE.